Small electrical motors are used in a number of aircraft applications. Although D.C. power supplies are readily available on such aircraft, D.C. motors are not in favor due to the rapid wear through erosion of the brushes therefor at high altitudes. In addition, the arcing of D.C. motor brushes causes R.F. interference and the speed of D.C. motors is not constant. There has accordingly been a move to replace small D.C. motors on aircraft with A.C. synchronous motors powered by inverters, that is, D.C. to A.C. converters. The speed of these synchronous motors is dependent only upon the frequency of the A.C. power supply which can be easily frequency regulated by modern techniques. Although not speed sensitive to voltage changes, the wide variation of D.C. primary voltage generated by aircraft voltage generators can cause A.C. motors powered from the D.C. voltage source through an inverter to be dangerously overpowered. This could result in catastrophic overheating of the motor windings. It is thus necessary to either regulate the voltage source or otherwise limit the power delivered to the A.C. motor.